Relaxin is a heterodimeric peptide hormone composed, in its mature form, of an A chain and a B chain linked via a disulphide bridge. Relaxin is produced as a prohormone with a third amino acid (C) chain in the configuration B-C-A. Relaxin is a member of a protein hormone superfamily which also includes insulin, insulin-like grown factors I and II (IGF-I and IGF-II), and the insulin-like hormones INSL3, 4, 5 and 6. The relaxin superfamily members have a wide range of biological activities which are well described in the art.
Relaxin has been conserved through vertebrate evolution and has been characterised in a large and diverse range of vertebrate species. In particular the cysteine residues in the B and A chains responsible for the intra- and inter-chain disulphide bonds are highly conserved. Whilst in most species only two forms of relaxin have been identified (relaxin and relaxin-3), in humans three distinct forms of relaxin have been described and the genes and polypeptides characterised. These have been designated H1, H2 and H3. Homologues of H1 and H2 relaxin have been identified in other higher primates including chimpanzees, gorillas and orangutans.
Of the three forms of relaxin in humans, the polypeptide encoded by the H2 gene is the major stored and circulating form. H2 relaxin is the only form known to be secreted in the blood. H1 relaxin expression is largely restricted to the decidua, placenta and prostate, whilst H3 relaxin expression is predominant in the brain. The differing expression patterns for H1, H2 and H3 relaxin may suggest some differences in biological roles, however all three forms display similar biological activity as determined, for example, by their ability to stimulate cAMP activity in cells expressing relaxin receptors, and accordingly share many biological functions in common.
The biological functions of relaxin include an ability to inhibit myometrial contractions, to stimulate remodelling of connective tissue and to induce softening of the tissues of the birth canal. Additionally, relaxin increases growth and differentiation of the mammary gland and nipple and induces the breakdown of collagen, one of the main components of connective tissue. Although originally identified as a pregnancy hormone, it is now well established that relaxin has a variety of other important roles and acts as an endocrine and paracrine factor in a broad range of tissues. For example, relaxin causes a widening of blood vessels (vasodilatation) in the kidney, mesocaecum, lung and peripheral vasculature, which leads to increased blood flow or perfusion rates in these tissues. It also stimulates an increase in heart rate and coronary blood flow, and increases both glomerular filtration rate and renal plasma flow. The brain is another target tissue for relaxin where it has been shown to bind to receptors in the circumventricular organs to affect numerous activities including blood pressure, drinking, memory related functions and addictive behaviours.
Aberrant relaxin activity and/or expression is also implicated in a number of disorders and diseases such as, for example, cardiovascular diseases, renal diseases, fibrotic disorders (including cardiac fibrosis and fibrosis associated with airway remodelling), neurological disorders, immune diseases and endometrial and reproductive disorders. According there exists a number of important clinical applications of relaxin and relaxin agonists and antagonists. The administration of recombinant relaxin has been demonstrated to be effective in reversing collagen deposition in pulmonary, renal and cardiac fibrosis (see for example, Samuel et al., 2004, Endocrinology 145:4125-4133; Mookerjee et al., 2005, Ann N Y Acad Sci 1041:190-193; co-pending U.S. patent application Ser. No. 11/133,763).
The biological actions of relaxin are mediated through G protein coupled receptors (see Bathgate et al., 2006, Pharmacol Rev 58:7-31 for a review). To date, H1, H2 and H3 relaxins have been shown to primarily recognise and bind four receptors, LGR7 (RXFP1), LGR8 (RXFP2), GPCR135 (RXFP3) and GPCR142 (RXFP4). LGR7 is the most widely expressed of these receptors and binds each of H1, H2 and H3 with high affinity. H1 and H2 relaxin also bind LGR8. H3 relaxin binds GPCR135 and GPCR142 in addition to LGR7.
In view of the range of potential clinical applications of relaxin there is a continuing need for the development of novel polypeptides displaying relaxin activity, which polypeptides have improved or varied biological activity when compared to naturally occurring relaxin polypeptides and/or which display different receptor binding specificities to naturally occurring relaxin polypeptides.